Optical Fiber Ribbon

TECHNICAL FIELD

The present disclosure relates to an optical fiber ribbon. The present application claims priority from Japanese Patent Application No. 2020-175367 filed on Oct. 19, 2020, contents of which are incorporated by reference in its entirety.

BACKGROUND

ART In recent years, there is an increasing demand to increase density of an optical fiber cable. As an example of increasing the density, it is known to reduce an outer diameter of an optical fiber to be mounted in the optical fiber cable or to use a plurality of optical fibers having different outer diameters. Further, in order to improve workability and identifiability at the time of splicing, there has been an optical fiber ribbon in which a plurality of optical fibers are arranged in parallel and integrally coated (Patent Literatures 1 and 2). CITATION LIST Patent Literature Patent Literature 1: JP2014-238480A Patent Literature 2: JP2014-219496A

SUMMARY

OF INVENTION An optical fiber ribbon according to the present disclosure includes: a plurality of optical fibers arranged in parallel in a direction orthogonal to a longitudinal direction of the plurality of optical fibers; and a collective coating layer covering an outer periphery of each of the plurality of optical fibers, in which the collective coating layer includes a connected portion at which the optical fibers adjacent to each other are connected in at least a part of the plurality of optical fibers, an outer diameter of each of the plurality of optical fibers is 215 μm or less, in a cross-sectional view of each of the optical fibers, the collective coating layer includes a thick portion and at least two thin portions in each of which a thickness of the collective coating layer is smaller than the thickness of the collective coating layer in the thick portion, and a difference between the thickness of the collective coating layer in the thick portion and the thickness of the collective coating layer in the thin portion is 5 μm or more and 19 μm or less.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of an optical fiber ribbon according to a first embodiment of the present disclosure. FIG. 2 is a cross-sectional view of an optical fiber of the optical fiber ribbon shown in FIG. 1 . FIG. 3 is a schematic view showing a relationship between a pitch of the optical fiber ribbon according to the first embodiment and V-grooves of a fusion machine in a fusion step. FIG. 4 is a cross-sectional view of an optical fiber ribbon according to a first modification. FIG. 5 is a view showing an optical fiber ribbon according to a second modification in a longitudinal direction. FIG. 6 is a cross-sectional view of the optical fiber ribbon shown in FIG. 5 . FIG. 7 is a cross-sectional view of an optical fiber ribbon used in an evaluation experiment.

DESCRIPTION OF EMBODIMENTS

Problems to be Solved by Present Disclosure In a case in which the optical fiber ribbons are mounted in the optical fiber cable at high density, since the optical fiber ribbons tend to be in contact with each other, friction may occur between the optical fiber ribbons. When the friction occurs non-uniformly in a longitudinal direction of the cable, the cable tends to meander. Further, transmission characteristics of the cable at a low temperature tend to deteriorate. In particular, in a case in which outer diameters of a plurality of optical fibers are different, the cable is more likely to meander, and the transmission characteristics are more likely to deteriorate. The present disclosure provides an optical fiber ribbon that enables high-density mounting of optical fibers in an optical fiber cable and prevents occurrence of cable meandering. Description of Embodiment of Present Disclosure First, an embodiment of the present disclosure will be listed and described. (1) An optical fiber ribbon according to an aspect of the present disclosure includes: a plurality of optical fibers arranged in parallel in a direction orthogonal to a longitudinal direction of the plurality of optical fibers; and a collective coating layer covering an outer periphery of each of the plurality of optical fibers, in which the collective coating layer includes a connected portion at which the optical fibers adjacent to each other are connected in at least a part of the plurality of optical fibers, an outer diameter of each of the plurality of optical fibers is 215 μm or less, in a cross-sectional view of each of the optical fibers, the collective coating layer includes a thick portion and at least two thin portions in each of which a thickness of the collective coating layer is smaller than the thickness of the collective coating layer in the thick portion, and a difference between the thickness of the collective coating layer in the thick portion and the thickness of the collective coating layer in the thin portion is 5 μm or more and 19 μm or less. In the optical fiber ribbon of the present disclosure, the collective coating layer includes the thick portion and at least two thin portions, and the thickness of the collective coating layer is non-uniform. In particular, since the difference between the thickness of the collective coating layer in the thick portion and the thickness of the collective coating layer in the thin portion is as relatively large as 5 μm or more and 19 μm or less, a contact area between the optical fiber ribbons can be reduced. Therefore, friction is unlikely to occur between the optical fiber ribbons adjacent to each other, and even when the optical fiber ribbons are mounted in a cable at high density, cable meandering can be further prevented. Further, according to the present disclosure, since the outer diameter of each optical fiber of the optical fiber ribbon is 215 μm or less, the optical fiber ribbons can be mounted in the cable at high density. (2) A distance between centers of the optical fibers adjacent to each other may be 220 μm or more and 280 μm or less. According to the present disclosure, since the distance between the centers of the optical fibers adjacent to each other is 220 μm or more and 280 μm or less, a general splicing device can be used. Even if each of the optical fibers is small in diameter, it is not necessary to prepare a dedicated splicing device for small-diameter optical fibers, so that a versatile optical fiber ribbon can be provided. (3) The collective coating layer may include a non-connected portion at which the optical fibers adjacent to each other are not connected in at least a part of the plurality of optical fibers. The connected portion may be formed intermittently in the longitudinal direction. According to the present disclosure, since the non-connected portion is provided intermittently in the longitudinal direction, the optical fiber ribbon is easily deformed in a cross section perpendicular to the longitudinal direction. Accordingly, the optical fibers can be mounted in the optical fiber cable at high density. (4) The plurality of optical fibers may include a first optical fiber having a first outer diameter and a second optical fiber having a second outer diameter. According to the present disclosure, since the plurality of optical fiber ribbons include the first optical fiber having the first outer diameter and the second optical fiber having the second outer diameter, the optical fibers can be mounted in the optical fiber cable at high density while preventing the cable meandering. Advantageous Effects of Present Disclosure According to the present disclosure, an optical fiber ribbon that enables high-density mounting of optical fibers in an optical fiber cable and prevents occurrence of cable meandering can be provided. Details of First Embodiment of Present Disclosure Specific examples of an optical fiber ribbon according to a first embodiment of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to these examples and is defined by the scope of the claims, and is intended to include all modifications within the scope and meaning equivalent to the scope of the claims. FIG. 1 is a cross-sectional view perpendicular to a longitudinal direction of an optical fiber ribbon 1 A according to an embodiment of the present disclosure. As shown in FIG. 1 , the optical fiber ribbon 1 A includes a plurality of optical fibers 11 and a collective coating layer 20 that covers the plurality of optical fibers 11 . In this example, 12 optical fibers 11 A to 11 L are arranged in parallel in a direction orthogonal to the longitudinal direction of the optical fiber ribbon 1 A. The plurality of optical fibers 11 are arranged at regular intervals. Outer peripheries of the plurality of optical fibers 11 are covered with the collective coating layer 20 , and the entire optical fibers 11 are connected by the collective coating layer 20 . An outer diameter D of each of the plurality of optical fibers 11 is 215 μm or less. In this example, the outer diameter D of each optical fiber is 200 Each optical fiber 11 includes, for example, a glass fiber 12 including a core and a cladding, a primary resin layer 13 covering an outer periphery of the glass fiber 12 , and a secondary resin layer 14 covering an outer periphery of the primary resin layer 13 . The glass fiber 12 may include pure silica glass, silica glass doped with germanium, or silica glass doped with fluorine. The primary resin layer 13 may include a soft material having a relatively low Young's modulus, as a buffer layer. The secondary resin layer 14 may include a hard material having a relatively high Young's modulus, as a protective layer. The Young's modulus of the primary resin layer 13 at 23° C. is preferably 0.04 MPa or more and 0.8 MPa or less, more preferably 0.05 MPa or more and 0.7 MPa or less, and still more preferably 0.05 MPa or more and 0.6 MPa or less. Since the Young's modulus of the primary resin layer 13 is 0.04 MPa or more and 0.8 MPa or less, voids are less likely to be generated in the optical fiber. The Young's modulus of the secondary resin layer 14 at 23° C. is preferably 900 MPa or more, more preferably 1000 MPa or more, and still more preferably 1200 MPa or more. The Young's modulus of the secondary resin layer 14 at 23° C. may be 3000 MPa or less, 2500 MPa or less, 2000 MPa or less, or 1800 MPa or less. Since the Young's modulus of the secondary resin layer 14 is 900 MPa or more, lateral pressure resistance is likely to be improved. When the Young's modulus of the secondary resin layer 14 is 3000 MPa or less, since the secondary resin layer 14 has appropriate elongation at break, stripping is easily performed. The collective coating layer 20 includes connected portions 21 at each of which adjacent optical fibers are connected in at least a part of the plurality of optical fibers 11 . In this example, the connected portions 21 are disposed between all adjacent optical fibers. The connected portions 21 are disposed such that a distance P between centers of adjacent optical fibers is 220 μm or more and 280 μm or less. The collective coating layer 20 may contain, for example, an ultraviolet curable resin. A Young's modulus of the collective coating layer 20 at 23° C. is preferably 50 MPa or more and 900 MPa or less, and more preferably 100 MPa or more and 800 MPa or less, from the viewpoint of lateral pressure resistance and flexibility of the optical fiber ribbon. FIG. 2 is a cross-sectional view of one optical fiber 11 B among the plurality of optical fibers 11 in the optical fiber ribbon 1 A shown in FIG. 1 . Since configurations of other optical fibers 11 A and 11 C to 11 L are the same as the configuration of the optical fiber 11 B shown in FIG. 2 , repeated description will be omitted. As shown in FIG. 2 , in the cross-sectional view of the optical fiber 11 B, the collective coating layer 20 includes a thick portion 22 and at least two thin portions 23 . In other words, a thickness of the collective coating layer 20 is non-uniform on the outer periphery of the optical fiber 11 B. Here, the thickness of the collective coating layer 20 on the outer periphery of the optical fiber 11 B is measured at a total of five locations except for the connected portion 21 . Among the five locations, a portion around a location where the thickness of the collective coating layer 20 is largest is defined as the thick portion 22 , and a portion around a location where the thickness of the collective coating layer 20 is thinner than the thick portion 22 by 5 μm or more is defined as the thin portion 23 . Positions of the thick portion 22 and the thin portion 23 on the outer periphery of the optical fiber 11 B shown in FIG. 2 are merely examples, and the present disclosure is not limited thereto. Further, since the collective coating layer 20 covers the outer periphery of the optical fiber 11 B, there is no location where the optical fiber 11 B is exposed from the collective coating layer 20 . A thickness d 2 of the collective coating layer 20 in the thin portion 23 is smaller than a thickness d 1 of the collective coating layer 20 in the thick portion 22 . Specifically, a difference between the thickness d 1 of the collective coating layer 20 in the thick portion 22 and the thickness d 2 of the collective coating layer 20 in the thin portion 23 is 5 μm or more and 19 μm or less. The thickness d 1 of the collective coating layer 20 in the thick portion 22 is, for example, 20 μm or less. The thickness d 2 of the collective coating layer 20 in the thin portion 23 is, for example, 1 μm or more and 15 μm or less. In this example, the thickness of the collective coating layer between the optical fiber 11 B and the optical fiber 11 A or 11 C refers to a thickness excluding the connected portion 21 . Next, fusion splicing of the optical fiber ribbon 1 A will be described. In general, in a case of splicing the optical fiber ribbon and another fiber ribbon, the plurality of optical fibers can be wholly fusion-spliced by using a multi-optical fiber fusion splicer (not shown). FIG. 3 is a schematic view showing a relationship between a pitch of the optical fiber ribbon 1 A (distance P between the centers of the adjacent optical fibers) and a V-groove base 30 of the fusion machine. As shown in FIG. 3 , the fusion splicer includes the V-groove base 30 including a plurality of V-grooves 31 in which the plurality of optical fibers 11 are arranged. In this example, the 12 optical fibers 11 A to 11 L are respectively arranged in 12 V-grooves 31 A to 31 L. A pitch P 0 of the V-grooves 31 A to 31 L is 250 μm according to an international standard of an outer diameter of the optical fiber. At the time of fusion-splicing, the optical fibers 11 A to 11 L in a state in which the collective coating layer 20 is stripped are arranged above the V-groove base 30 . The optical fibers 11 A to 11 L are arranged such that, for example, center positions of the V-grooves 31 A to 31 L in a parallel arrangement direction and center positions of the optical fibers 11 A to 11 L in a parallel arrangement direction are aligned. In this state, a clamp cover (not shown) of the multi optical fiber fusion splicer is closed, and the optical fibers 11 A to 11 L are pushed down from above by the clamp cover. If there is no connected portion in the optical fiber ribbon and a distance between centers of adjacent optical fibers among the optical fibers is zero, the distance between the centers is smaller than the pitch P 0 of the V-grooves 31 A to 31 L. In this case, the plurality of optical fibers are arranged in a manner of being gathered toward a center position of the V-groove base 30 , and are not arranged in a manner of facing the V-grooves 31 A to 31 L. Accordingly, the plurality of optical fibers are not necessarily accommodated in the V-grooves 31 A to 31 L, respectively, and for example, a case in which the optical fibers are not accommodated in the V-grooves 31 A and 31 L occurs. This may also occur in a case in which the distance P between the centers of adjacent optical fibers is less than 220 μm. On the other hand, in the optical fiber ribbon 1 A in this example, since the connected portions 21 are arranged such that the distance P between the centers of adjacent optical fibers is 220 μm or more and 280 μm or less, the optical fibers 11 A to 11 L are arranged in a manner of facing the V-grooves 31 A to 31 L respectively. Therefore, when the optical fibers 11 A to 11 L are pushed down substantially vertically, the optical fibers 11 A to 11 L are accommodated in the V-grooves 31 A to 31 L respectively. FIG. 3 shows that the optical fibers 11 A to 11 L in a state in which the collective coating layer 20 is stripped are to be accommodated in the V-grooves 31 A to 31 L respectively, but for example, the primary resin layer 13 and the secondary resin layer 14 may be further stripped in addition to the collective coating layer 20 , and only the glass fiber 12 may be accommodated in each of the V-grooves 31 A to 31 L. As described above, in the optical fiber ribbon 1 A according to this example, the collective coating layer 20 includes the thick portion 22 and at least two thin portions 23 , and the thickness of the collective coating layer 20 is non-uniform. In particular, since the difference between the thickness d 1 of the collective coating layer 20 in the thick portion 22 and the thickness d 2 of the collective coating layer 20 in the thin portion 23 is as relatively large as 5 μm or more and 19 μm or less, even when a plurality of optical fiber ribbon 1 A are mounted in an optical fiber cable at high density, a contact area between the optical fibers in adjacent optical fiber ribbons 1 A and a contact area between the optical fiber ribbons can be reduced. Accordingly, friction is less likely to occur between adjacent optical fiber ribbons, cable meandering can be prevented, and cable laying work can be improved. In the optical fiber ribbon 1 A of this example, the collective coating layer 20 covers the outer peripheries of the plurality of optical fibers. If the outer periphery of any optical fiber 11 is not completely covered with the collective coating layer 20 and there is a portion exposed from the collective coating layer 20 , the collective coating layer 20 may be peeled off from the optical fiber 11 starting from the exposed portion, and as a result, the optical fiber 11 may be single core-separated from the optical fiber ribbon. However, in this example, since the outer peripheries of the plurality of optical fibers are covered with the collective coating layer 20 , the single core-separation does not occur. Since the outer diameter of each optical fiber 11 of the optical fiber ribbon 1 A of this example is 215 μm or less, a cross-sectional area of the optical fiber 11 is small, and the optical fiber ribbons 1 A can be mounted in an optical fiber cable at high density. In this example, since the distance between the centers of adjacent optical fibers is 220 μm or more and 280 μm or less, the optical fiber ribbon can be used in a general multi-core fusion machine or splicing device. Even if each of the optical fibers 11 is small in diameter, it is not necessary to prepare a multi-core fusion machine dedicated to small-diameter fibers, such as a multi-core fusion machine in which V-grooves are formed at a narrow pitch, so that versatility of the optical fiber ribbon 1 A is high and a manufacturing cost can be reduced. (First Modification) In the optical fiber ribbon 1 A according to the first embodiment, the outer diameters of the optical fibers 11 are all the same, but the outer diameters of the optical fibers 11 may not be the same. FIG. 4 is a cross-sectional view perpendicular to a longitudinal direction of an optical fiber ribbon 1 B according to a first modification. In the description of FIG. 4 , elements substantially the same as or corresponding to those in the configuration shown in FIG. 1 are denoted by the same reference numerals, and repeated description thereof will be omitted. As shown in FIG. 4 , the plurality of optical fibers 11 of the optical fiber ribbon 1 B include first optical fibers each having an outer diameter D 1 (an example of a first outer diameter), and second optical fibers each having an outer diameter D 2 (an example of a second outer diameter). In this example, the outer diameter D 1 of the first optical fibers 11 A′, 11 C′, 11 E′, 11 G′, 11 r , and 11 K′ is different from the outer diameter D 2 of the second optical fibers 11 B, 11 D, 11 F, 11 H, 11 J, and 11 L, and the first optical fibers each having the outer diameter D 1 and the second optical fibers each having the outer diameter D 2 are alternately arranged. The outer diameter D 1 of the first optical fiber and the outer diameter D 2 of the second optical fiber are both 215 μm or less. In this example, the outer diameter D 1 is 200 and the outer diameter D 2 is 180 Each of the first optical fibers 11 A′, 11 C′, 11 E′, 11 G′, 11 r , and 11 K′ includes the glass fiber 12 , the primary resin layer 13 , and the secondary resin layer 14 . Further, when the outer diameters D 1 and D 2 are different from each other, the connected portions 21 of the collective coating layer 20 are also arranged such that the distance P between the centers of adjacent optical fibers is 220 μm or more and 280 μm or less. Further, the collective coating layer 20 includes the thick portion 22 and at least two thin portions 23 , similar to the optical fiber ribbon 1 A. When the optical fiber ribbon 1 B according to the first modification is mounted in an optical fiber cable, the second optical fibers 11 B, 11 D, 11 F, 11 H, 11 J, and 11 L each having the small outer diameter D 2 are disposed in gaps between the first optical fibers 11 A′, 11 C′, 11 E′, 11 G′, 11 r , and 11 K′ each having the large outer diameter D 1 . Accordingly, as compared with a case in which the plurality of optical fibers 11 have the same outer diameter, mounting density of the optical fibers 11 with respect to the optical fiber cable is higher in a case in which the plurality of optical fibers 11 have the outer diameters D 1 and D 2 different from each other. Furthermore, the thick portion 22 of each optical fiber may be disposed at a position facing the thin portion 23 of another optical fiber. Accordingly, as compared with a case in which the collective coating layer 20 has a uniform thickness, the mounting density of the optical fiber 11 with respect to the optical fiber cable is higher in a case in which the thick portion 22 and the thin portion 23 are provided. As described above, since the optical fiber ribbon 1 B according to the first modification includes the plurality of optical fibers 11 including the first optical fibers each having the outer diameter D 1 , and the second optical fibers each having the outer diameter D 2 , the optical fibers 11 can be mounted in the optical fiber cable at higher density. (Second Modification) The connected portions 21 of the optical fiber ribbon 1 A according to the first embodiment connect all adjacent optical fibers, but the arrangement of the connected portions is not limited thereto. FIG. 5 is a view showing an optical fiber ribbon 1 C according to a second modification in a longitudinal direction. FIG. 6 is a cross-sectional view of the optical fiber ribbon 1 C. In the description of FIG. 6 , elements substantially the same as or corresponding to those in the configuration shown in FIG. 1 are denoted by the same reference numerals, and repeated description thereof will be omitted. As shown in FIGS. 5 and 6 , a collective coating layer 20 C of the optical fiber ribbon 1 C includes non-connected portions 24 at each of which adjacent optical fibers are not connected in at least a part of the plurality of optical fibers 11 . In the second modification, the non-connected portions 24 are formed between the optical fibers 11 A and 11 B, between the optical fibers 11 C and 11 D, between the optical fibers 11 D and 11 E, between the optical fibers 11 F and 11 G, between the optical fibers 11 G and 11 H, between the optical fibers 11 I and 11 J, and between the optical fibers 11 J and 11 K. The arrangement positions of the non-connected portions 24 shown in FIG. 6 is an example, and are not limited thereto. The non-connected portions 24 are intermittently formed in the longitudinal direction of the optical fiber ribbon 1 C. Even when the non-connected portions 24 are formed, the optical fibers 11 A to 11 L are also arranged such that the distance P between centers of adjacent optical fibers is 220 μm or more and 280 μm or less. As described above, according to the optical fiber ribbon 1 C of the second modification, since the non-connected portions 24 are intermittently formed in the longitudinal direction of the optical fiber ribbon 1 C, deformability of the optical fiber ribbon 1 C is enhanced in a cross section perpendicular to the longitudinal direction. Since the optical fiber ribbon 1 C is easily deformed, a plurality of optical fiber ribbons 1 C can be mounted in an optical fiber cable at high density. Although outer diameters of the optical fibers 11 of the optical fiber ribbon 1 C according to the second modification are all the same, the optical fibers 11 of the optical fiber ribbon 1 C may have different outer diameters D 1 and D 2 as shown in the first modification. Evaluation Experiments A low-temperature characteristic and the presence or absence of single core-separation of the optical fiber ribbon of the present disclosure were evaluated. FIG. 7 is a cross-sectional view perpendicular to a longitudinal direction of an optical fiber ribbon 1 X used in an evaluation experiment. In the description of FIG. 7 , elements substantially the same as or corresponding to those in the configuration shown in FIG. 4 or 6 are denoted by the same reference numerals, and repeated description thereof will be omitted. As shown in FIG. 7 , the optical fiber ribbon 1 X used in the evaluation experiment includes four optical fibers 11 A′, 11 B, 11 C′, and 11 D. Each of the optical fibers includes the glass fiber 12 , the primary resin layer 13 , and the secondary resin layer 14 . The outer diameter D 1 of each of the optical fiber 11 A′ and 11 C′ is 200 μm. The outer diameter D 2 of each of the optical fibers 11 B and 11 C is 180 In other words, the optical fiber having the outer diameter D 1 of 200 μm and the optical fiber having the outer diameter D 2 of 180 μm are alternately arranged along a direction orthogonal to the longitudinal direction of the optical fiber ribbon 1 X. In manufacturing steps of the optical fiber ribbon 1 X, outer peripheries of the four optical fibers 11 A′ to 11 D arranged in parallel are coated with an ultraviolet curable resin. Thereafter, the ultraviolet curable resin is cured by being irradiated with ultraviolet rays, and the collective coating layer 20 C is formed. At this time, the outer peripheries of the optical fibers 11 A′ to 11 D are covered with the collective coating layer 20 C. The thick portion 22 and the thin portion 23 are formed by adjusting a thickness of the applied ultraviolet curable resin by a shape of a die or the like. Further, after the ultraviolet curable resin is cured and the connected portions 21 are formed between all the adjacent optical fibers, the non-connected portions 24 are formed by inserting a cleaving blade such as a cutter between the adjacent optical fibers 11 A′ and 11 B and between the adjacent optical fibers 11 C′ and 11 D intermittently in the longitudinal direction of the optical fiber ribbon 1 X. The connected portion 21 remains between the optical fibers 11 B and 11 C′ where no cleaving blade is inserted. In the evaluation experiment, samples No. 1 to No. 5 for the optical fiber ribbon 1 X including various thicknesses of the collective coating layer 20 C were prepared by adjusting the thickness of the applied ultraviolet curable resin. Samples No. 6 to No. 8 were prepared as comparative examples. In the evaluation experiment, the optical fiber 11 A′ was selected as any optical fiber of each sample, and the thickness of the collective coating layer 20 C was measured at any eight measurement positions I to VIII in the optical fiber 11 A′. Further, the optical fiber ribbon 1 X was mounted on an optical fiber cable, and the low-temperature characteristic of the optical fiber ribbon 1 X was evaluated. Here, the low-temperature characteristic was evaluated by measuring an attenuation amount per unit distance in a case in which light having a wavelength of 1.55 μm was incident on the fiber 11 A′ inside the cable under an environment of 23° C. and an environment of −30° C., and evaluating a difference between measured values under the two temperature environments. Further, the optical fiber ribbon 1 X was taken out from the optical fiber cable, and the presence or absence of the single core-separation of the optical fiber was examined. Evaluation results thereof are shown in Table 1. TABLE 1 Sample No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 Thickness of I 20 20 20 20 20 20 20 20 collective II 20 20 5 1 15 0 17 5 coating III 20 20 20 20 20 20 20 20 layer (μm) IV 5 5 5 1 15 0 17 20 V 20 20 20 20 20 20 20 20 VI 5 5 5 1 15 20 17 20 VII 20 20 20 20 20 20 20 20 VIII 20 5 5 1 15 20 17 20 Low-temperature 0.3 dB/ 0.3 dB/ 0.3 dB/ 0.3 dB/ 0.3 dB/ 0.3 dB/ Exceed Exceed characteristic km or km or km or km or km or km or 0.3 dB/ 0.3 dB/ less less less less less less km km Presence or absence Absence Absence Absence Absence Absence Presence Absence Absence of optical fiber separation As shown in Table 1, in the samples No. 1 to No. 5, the optical fiber separation of the optical fiber ribbon ix was not found. In particular, in the sample No. 4, the fiber separation was not found. In the sample No. 4, the measurement positions II, IV, VI, and VIII correspond to the thin portion 23 , and the thickness of the collective coating layer 20 C at each measurement position is 1 μm. On the other hand, in the sample No. 6 as the comparative example, the single core-separation was found. The sample No. 6 includes two portions where the thicknesses of the collective coating layer 20 C are zero at the measurement positions II and IV. Portions at which the optical fiber 11 A′ is not covered with the collective coating layer 20 C and is exposed from the collective coating layer 20 C are at the measurement positions II and IV. It was confirmed that the collective coating layer 20 C was peeled off from the optical fiber 11 A′ starting from the exposed portion, and as a result, the optical fiber 11 A′ was separated from the optical fiber ribbon ix. From the above, it was confirmed that when the outer peripheries of the plurality of optical fibers 11 were covered with the collective coating layer 20 C, there was no fiber separation. In the sample No. 4, the measurement positions I, III, V, and VII correspond to the thick portion 22 , and the thickness of the collective coating layer 20 C at each measurement position is 20 μm. The thickness of the collective coating layer 20 C in the thin portion 23 of the sample No. 4 was 1 μm. It was confirmed that cable attenuation of the sample No. 4 was 0.3 dB/km or less. From the above, it was confirmed that the optical fiber ribbon ix having low cable attenuation can be implemented when the difference between the thickness of the collective coating layer 20 C in the thick portion 22 and the thickness of the collective coating layer 20 C in the thin portion 23 is 19 μm or less. As shown in Table 1, the cable attenuation of the samples No. 1 to No. 5 were all 0.3 dB/km or less, and it was confirmed that the low-temperature characteristic was good. In particular, in the sample No. 1, the thicknesses of the collective coating layer 20 C at the measurement positions IV and VII corresponding to the thin portions 23 are 5 μm. On the other hand, the cable attenuation of the sample No. 8 as the comparative example exceeded 0.3 dB/km. In the sample No. 8, the thickness of the collective coating layer 20 C at the measurement positions II corresponding to the thin portion 23 is 5 μm. In other words, the cable attenuation of the sample No. 1 including two thin portions 23 was small, whereas the cable attenuation of the sample No. 8 including only one thin portion 23 was larger than that of the sample No. 1. As described above, it was confirmed that when the collective coating layer 20 C of each optical fiber 11 includes at least two thin portions, the optical fiber ribbon ix having the small cable attenuation can be implemented. As shown in Table 1, it was confirmed that the cable attenuation of the sample No. 5 was 0.3 dB/km or less, whereas the cable attenuation of the sample No. 7 exceeded 0.3 dB/km. In the samples No. 5 and No. 7, the measurement positions I, III, V, and VII correspond to the thick portion 22 , and the measurement positions II, IV, VI, and VIII correspond to the thin portion 23 . In the sample No. 5, the thickness of the collective coating layer 20 C in the thick portion 22 is 20 μm, and the thickness of the collective coating layer 20 C in the thin portion 23 is 15 μm. On the other hand, in the sample No. 7, the thickness of the collective coating layer 20 C in the thick portion 22 is 20 μm, and the thickness of the collective coating layer 20 C in the thin portion 23 is 17 μm. In the sample No. 7, since the difference between the thickness of the collective coating layer 20 C in the thick portion 22 and the thickness of the collective coating layer 20 C in the thin portion 23 is not sufficient, the friction between the optical fibers cannot be reduced. As a result, it was confirmed that the cable attenuation increased. From the above, it was confirmed that the optical fiber ribbon ix having low cable attenuation can be implemented when the difference between the thickness of the collective coating layer 20 C in the thick portion 22 and the thickness of the collective coating layer 20 C in the thin portion 23 is 5 μm or more. Although the present disclosure has been described in detail with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present disclosure. The numbers, positions, shapes or the like of components described above are not limited to the above embodiment, and can be changed to suitable numbers, positions, shapes or the like during carrying out the present disclosure. REFERENCE SIGNS LIST 1 A, 1 B, 1 C, 1 X optical fiber ribbon 11 , 11 A to 11 L optical fiber 12 glass fiber 13 primary resin layer 14 secondary resin layer 20 C collective coating layer 21 connected portion 22 thick portion 23 thin portion 24 non-connected portion 30 V-groove base 31 , 31 A to 31 L V-groove D, D 1 , D 2 outer diameter of optical fiber P distance between centers of adjacent optical fiber ribbons P 0 V-groove pitch d 1 thickness of collective coating layer in thick portion d 2 thickness of collective coating layer in thin portion